The CdS/CdTe interface was investigated by the free-electron laser (FEL) internal photoemission technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found two thresholds in the photocurrent spectrum, which can be identified as steplike band discontinuities. It is demonstrated that there is a mixed crystal layer ofCdS1−xTexat the CdS/CdTe interface. ©1997 American Institute of Physics.